The present invention relates to the process chemistry during pressure oxidation of molybdenite concentrates and more particularly to a process for controlling and optimizing the process chemistry during the pressure oxidation of molybdenite concentrates.
Extraction of molybdenum from molybdenite-containing materials by an aqueous process has been the subject of study for almost 50 years. In 1952, E. S. Usataya1 reported on the oxidation of molybdenite in water solutions. He found that in neutral, acidic, or weakly alkaline solutions the decomposition products precipitate on the molybdenite surface and protect the molybdenite from further oxidation. He found that strong bases and strong oxidizing agents impede the formation of the protective layers. Increasing temperature accelerated the oxidation rate in alkaline solutions, but up to 60xc2x0 C. had no effect in acidic solutions.
1Usataya, E. S., xe2x80x9cOxidation of molybdenite in water solutions,xe2x80x9d Zapiski Vsesoyuz Mineral Obshschestva, v 81, 298-303 (1952). 
A Japanese patent2 was issued for oxygen pressure oxidation (POX) of molybdenite in 1962. The example in this patent leached a 55.5% Mo, 36.4% S, and 4.4% Cu concentrate at 9% solids at 200xc2x0 C. and 200 atmospheres oxygen. The molybdic acid precipitate that formed during leaching was dissolved using ammonia for recovery of an ammonium molybdate. The reaction formula was:
2 MoS2+6H2O+9O2xe2x86x922H2MoO4↓+4H2SO4xe2x80x83xe2x80x831)
2Sada, Koji, xe2x80x9cExtraction of molybdenum,xe2x80x9d Japanese patent 15.207(""62), assigned to Awamura Mining Co., Ltd. 
In 1971, AMAX patented similar reactions3 but chose a temperature greater than 80xc2x0 C. and a pressure greater than 3.5 atmospheres. In a second AMAX patent4, alkali hydroxide or ammonium hydroxide was added continuously to neutralize the acid as it formed and maintain the pH at 7-12.
3Barry, Henry F. et al, xe2x80x9cAqueous phase oxidation of molybdenum disulfide,xe2x80x9d German patent 2,043,874 (1971). 
4Hallada, Callvin J., et al., xe2x80x9cConversion of molybdenum disulfide to molybdenum oxide,xe2x80x9d German patent 2,045,308 (1971). 
Mel""nikov5 in 1970 and K. Ya. Shapiro et al.6 in 1973 reported on pressure oxidation in the presence of sulfuric acid. Shapiro postulated the soluble molybdenum species as MoO2SO4. Mel""nikov used an oxidized concentrate containing 17% Mo and 15% Fe, at 16% solids and 100xc2x0 C. Shapiro""s conditions for a 47% Mo concentrate were 9% solids, 225xc2x0 C., and less than 10 atmospheres oxygen pressure, for three hours.
5Shapiro, K. Ya, et al., xe2x80x9cSulfuric acid processing of oxidized molybdenum raw materials,xe2x80x9d Nauk. Tr. Vses. Nauch.-Issled. Proekt. Inst. Tugoplavkikh Metal. Tverd. Splavov, 1970. 
6Mel""nikov, B. S. et al., xe2x80x9cOxidative water-autoclave leaching of molybdenum concentrate,xe2x80x9d Nauchn Tr., Vses. Nauchno-Issled. Proektn. Inst. (1973) 14, 251-157. 
A 1970 paper by Irkov7 reports the solubility of molybdic oxide in sulfuric acid solutions at 22, 50, 71, and 89xc2x0 C. The paper shows molybdenum solubility decreasing with increasing temperature and the effect of acid concentrations above 20 percent.
7Irkov, F. Ya, Palant, A. A., and Reznichenko, V. A., xe2x80x9cSolubility of anhydrous molybdenum (VI) oxide in aqueous solutions of sulfuric acid,xe2x80x9d Russian Journal of Inorganic Chemistry, v 15, 5, 695-697 (1970). 
A 1975 paper by Shapiro and B. S. Mel""nikov8 reports on POX decomposition studies of 1) pure molybdenite, 99.5% MoS2, 2) high grade concentrate, 47% Mo, and 3) a low grade industrial product containing 16% Mo and 15% Fe. The leaching conditions were 9% solids, less than 160 to 225xc2x0 C., 5 to 20 atmospheres oxygen overpressure, and 15 to 195 minutes.
8Mel""nikov, B. S. and Shapiro, K. A., xe2x80x9cWater-autoclave decomposition of molybdenite raw material,xe2x80x9d Protsessy Poluch. Rafinirovaniya Tugoplavkikh Met. (1975) 113-120, 253-260. 
The paper shows the effect of temperature on the oxidation rate with 15 atmospheres oxygen overpressure. The oxidation rate was rapid above 190xc2x0 C. The paper also shows the effect of oxygen overpressure on the oxidation rate at 225xc2x0 C. The oxidation rate was directly proportional to the oxygen overpressure. The authors postulate that the oxidation rate as affected by temperature and oxygen overpressure controls the amount of soluble molybdenum.
The paper also includes the effect of adding iron to the high grade molybdenite. The addition of iron greatly accelerates the oxidation of the molybdenite. The temperature effect on the rate is less with the added iron. With iron present, the molybdenum in solution reaches a peak near the 60% molybdenite oxidation point and then decreases to a level about twice that with no iron added. The authors postulate the formation of a molybdenum-iron heteropoly complex that decomposes as the acid concentration increases. It also may be a ferrous complex that decomposes as the soluble iron is oxidized to ferric.
Oxidation of the low-grade industrial-grade concentrate was also studied and the effect of temperature (150-225xc2x0 C.) on the rate of oxidation, the solubility of molybdenum, the iron and sulfuric acid concentrations at less than 15 atmospheres oxygen overpressure is shown. The data clearly illustrate an incubation period while sufficient pyrite/chalcopyrite is oxidized to generate sufficient sulfuric acid to begin the molybdenite oxidation. Rapid oxidation of the molybdenite did not occur until the soluble iron was about 8 g/l and the sulfuric acid concentration was about 40 g/l. The data indicate that batch testing using water as the lixiviant would not produce data that could be translated to a continuous reactor.
Rothmann9 in 1980 and Bauer10 in 1984 patented an oxygen pressure leaching processes in which molybdenite was added to a reacted mixture containing up to 100 g/l sulfuric acid. The leaching was done at 250 to 300xc2x0 C. and 10 to 20 atmospheres-oxygen overpressure.
9Rothmann, Hans W. and Bauer, Guenter, xe2x80x9cMethod for recovering molybdenum oxide,xe2x80x9d German patent DE 2,830,394 (1980). 
10Bauer, Guenter and Eckert, Joachim, xe2x80x9cRecovery of molybdenum oxide,xe2x80x9d German patent DE 3,128,921 (1983). 
In 1986, Gock11 patented the oxidation of molybdenite (97.5%) while not completely oxidizing the contained flotation reagents. This would keep the molybdenum as soluble, reduced-valence species, not hexavalent species that would precipitate. The conditions (acid lixiviant of 0.2 to 0.6 pH, 160-220xc2x0 C., and oxygen overpressure) do not indicate how the reaction control can prevent complete oxidation of the flotation reagents.
11Gock, Eberhard, xe2x80x9cPure molybdenum trioxide produced by way of extraction of molybdenum from sulfate solutions,xe2x80x9d German patent DE 84 3,443,806 (1986). 
In 1998, Cyprus Amax12 patented a process in which molybdenite is added to the filtrate from a prior reaction and allowed to react to 70 to 95% oxidation. Unreacted molybdenite is recovered from the solids by froth flotation. The unreacted molybdenite and some residual liquid are recycled to the leaching reactor. Oxidation conditions are 175-225xc2x0 C. and 50 to 300 psig oxygen overpressure. The flotation tails, molybdic oxide and/or molybdic acid also contain a small amount of molybdenite and must be calcined prior to sale as molybdic oxide. The balance of the residual liquid from the autoclave is treated for recovery of rhenium and copper.
12W. H. Sweetser and L. N. Hill, xe2x80x9cProcess for autoclaving molybdenum sulfide,xe2x80x9d U.S. Pat. No. 5,804,151 (1998). 
A Canadian patent by Brassier and Pascal13 followed the formation of heteropoly compounds when leaching crude calcium molybdate precipitates with sulfuric acid. The data in this patent indicated significant formation of heteropoly phosphates and silicates when leaching was done at less than 0.5 molar sulfuric acid (49 g/l). When leaching was done at 1 molar sulfuric acid, the quantities of heteropolys were reduced by more than one order of magnitude. The inference from these data is that the initial leach feed must contain sufficient free acid to avoid the low acid range where the heteropolys form.
13Brassier, Cecile and Baron, Pascal: Canadian Patent 2, 154,133 (1996) also French Patent 2,272,701. xe2x80x9cProcess for the recovery of molybdenum from impure calcium molybdate resulting from the treatment of uraniferous oresxe2x80x9d 
It is clear from the various processes, conditions, and theories used over the years that a fundamental understanding of the process chemistry involved in oxidation of molybdenite concentrates is still needed in order to reliably control the chemical reactions in an oxygen pressure leach and to predict the concentration of soluble molybdenum present in the autoclave discharge slurries after pressure oxidation.
It is therefore an object of the present invention to provide a leaching model which predicts the amount of soluble molybdenum present in the autoclave discharge slurries after the pressure oxidation of molybdenite concentrates.
It is a further object of the present invention to provide a process for optimizing and controlling the process chemistry during the pressure oxidation of molybdenite concentrates.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
The present invention provides a leaching model which predicts the amount of soluble molybdenum present during the pressure oxidation of molybdenite concentrates and provides a process for optimizing and controlling the process chemistry during the pressure oxidation in the autoclave.
The process chemistry during the pressure oxidation of molybdenite concentrates is controlled, to enhance recovery in accordance with the present invention, by regulating the amount of ferric iron and excess sulfuric acid in the final leach slurry. The oxidation kinetics are controlled by the temperature and oxygen overpressure used in the autoclave. The model predicts the soluble molybdenum based on the concentrate analysis, the recycle solution analysis, and the pulp density used. Good oxidation kinetics were found to require temperatures greater than 200xc2x0 C. and greater than 50 psig oxygen overpressure.
The model is able to determine conditions of molybdenum solubility independent of molybdenum oxidation. Using the computer model, greater than 99% oxidation of the molybdenite was achieved while producing a final slurry where less than 20% or more than 80% of the molybdenum was soluble. Dissolution of the copper content was greater than 99% and dissolution of the iron was 60 to 90% from the pyrite and chalcopyrite present in the concentrate. Substantial quantities of this iron could then precipitate as iron molybdate. By leaching under high excess acid conditions, the amount of soluble silicon could be held under 100 mg/l. Most of the rhenium, arsenic, and phosphorus were dissolved regardless of the leaching conditions.
Other objects, features and advantages of the invention will be apparent from the following description of preferred embodiments thereof, including illustrative non-limiting examples, of the modeling process and the resulting product composition.